Reed switch having multi-layer diffused contacts



y 6 TAKESHI sAsAMoTo ETAL 3,249,728

REED SWITCH HAVING MULTI-LAYER DIFFUSED CONTACTS Filed July 29, 1963 2Sheets-Sheet 1 CONTACT RESISTANCE lNmn o.I I 5 I0 so a0 9095 99 99.9

CUMULATIVE DISTRIBUTION IN PER CENT A. MULTI LAYER DIFFUSED CONTACTS(NICKEL-GOLD) A2 MULTI LAYER DIFFUSED CONTACTS (COPPER-GOLD) INV N"ORS BGOLD DIFFUSED CONTACTS TAKESHIESAESAMOTO (MEASURED AT O.9mv I000 /s) BYYosHITAMI OHKI ATTORNEYS y 1966 TAKESHI SASAMOTO ETAL 3,249,728

REED SWITCH HAVING MULTI-LAYER DIFFUSED CONTACTS Filed July 29, 19638220550 xmfiw gorgi miwg mm mmom OmOh Om OmON O. m

m O m 0 SNOliVHHdO d0 SNOII'IIW 2.03.200 owwnuuzo 0400 m ATTORN EYSINVENTORS TAKESHI SASAMOTO BY YOSHITAMI O Kl mm mm om omohowomodvom om om N M X m 1 m m s w 0 J.- m 0 O 0 3 H. V U 3% s 0m 0w m5 om m L I3,249,728 REED SWITCH HAVING MULTI-LAYER DIFFUSED CONTACTS TakeshiSasamoto and Yoshitami Ohki, Tokyo, Japan, assignors to Nippon ElectricCompany Limited, Tokyo, Japan,'a corporation of Japan Filed July 29,1963, Ser. No. 298,064 Claims priority, application Japan, Aug. 1, 1962,

3 Claims. ci. 200-166) This invention relates to improvements in theconstruction of the contacts of sealed metallic contact devices and isparticularly useful with reed switches of the type in which reeds offerromagnetic material are caused to operate by means of an externallyapplied magnetic field so.

United States Patent size, which requires that they be made of an alloywith resistance of the alloy itself is inherently high and further,

the alloy oxidizes rather quickly.

In order to obviate these disadvantages, a method has for some time beenknown by which the contacts of the reeds are plated with a noble metalsuch as gold and then subjected to a diffusion process in a reducingatmosphere such as for example, hydrogen, so that the iron and nickel ofwhich the reed core is composed may diffuse into the gold layer. Howeverthe iron which diffuses into the newly created layer composed of gold,nickel, and iron is easily oxidized in a brief time period prior to thesealing process to form an oxide film with high contact resistance Evenafter the reeds have been sealed over the layer. in an envelope, ironcombines with residual oxygen within the envelope onaccount of localheating accompanied by mechanical collision occurring in the operationof the reed switch, with the result that both the contact surfaces ofthe reeds and the powders produced by mechanical wear are oxidized tobring about an increase in the contact resistance.

Another difliculty with the use of iron is its inherent quality ofeasily establishing fine bridging in switching a current from on to off.This tendency holds true for iron present in proximity to the surface ofthe abovementioned newly created layer of gold, nickel, and iron. As aresult, with reed switches using gold diffusion contacts, fine bridgingmay occur in a comparatively early state of operation, and willculminate in the inability of the contacts to separate.

A still further problem with reed switches using golddiflusion contactsas described above results from the formation of metallic oxide powdersdue to chemical combination of the powder with residual oxygen withinthe envelope when current is switched from on to off and vice versa.These powders tend to accumulate and adhere to the contact surface,resulting in imperfect contact, under circuit conditions in which thepossibility of occurrence of glow discharge is small. a

3,249,728 Patented May 3, 1966 Accordingly, it is an object of thisinvention to provide a sealed metallic contact device having a longeruseful switching life than similar prior art devices.

It isanother object of the invention to provide a reed switch having alower contact resistance during its life span than such switches ofknown construction.

All of the objects, features and advantages of this invention and themanner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawing, in which FIG. 1 shows a longitudinal crosssectional view of a reed switch in accordance with an embodiment of theinvention;

FIG. 2 shows an enlarged view of the free end portion of the reed in theembodiment of FIG. 1;

FIG. 3 illustrates a lateral cross sectional view of the free endportion of the reed before it is subjected to the diffusion process; and

FIGS. 4, 5 and 6 are graphs illustrating the superior advantagesrealized by reason of the invention.

In accordance with the invention a switch of improved characteristics isachieved by providing contacts made by means of the gold diffusion typeprocess referred to above but in which the amount of iron in the goldouter layer is substantially reduced. This is accomplished by providingan intermediate layer of suitable metal between the reed core materialand the gold layer.

Referring now to FIG. 1 there is shown a longitudinal cross section of amake-type reed switch, wherein the numerals 1 and 2 denote reeds made ofnickel-iron alloy, and numeral 3 denotes an envelope of suitablematerial such as glass or ceramic. The reeds 1 and 2 are separatedattheir free ends by a small gap 4, with the result that they attracteach other to make contact to close an electrical circuit when amagnetic field is applied in the axial direction of the reeds. Uponremoval of the magnetic field, the reeds 1 and 2 are released to thenormal position by'the spring restoring force to interrupt theelectrical circuit.

FIG. 2 shows an enlarged view of the free end of the reed 1, the endportion 8 being the part to be gold plated and serving as one of thereed contacts. FIG. 3 shows a cross section of the portion 8, takenalong the line 3-3 of FIG. 2, and illustrates the construction of thisportion prior to the diffusion process. In FIG. 3, the numeral 5 denotesthe inner core preferably of nickel-iron alloy. In accordance with theinvention, an intermediate layer 6 is provided over the core 5; thislayer 6 is composed of a metal or metals other than iron and gold suchas, for example, nickel, cobalt, manganese, or copper, etc., that candiffuse with gold. The numeral 7 denotes a gold layer.

After the end portion of each reed which forms the contact portion suchas shown in FIG. 3 has been plated, the reeds 1 and 2 are subjected tothe diffusion process at a suitable high temperature.

As one example, we have achieved good results when the intermediatelayer 6 is made of nickel or copper and has a thickness of approximately0.002 mm., which may be nearly equal that of the outer gold layer 7,however, this ratio is not critical and various thicknesses and ratiosmay be employed. Diffusion is achieved by heating to a temperature of850 C. for 15 minutes in a non-inert atmosphere such as hydrogen.However, it should be observed that the temperature and heating timesare not critical; the temperature may be in the range of 650 to 950 C.and the time may vary from 1 minute to minutes, depending upon thetemperature. The temperature, however, must not exceed the melting pointof i any of the materials forming the core and layers 6 and 7. Thisheating process causes gold to diffuse into the intermediate layer 6 andreed core 5, further causing the intermediate layer metal to diffuseinto the gold layer 7, and resulting in a reduction of the proportion ofiron on the contact surfaces compared with the prior art process. Thisresults in considerable reduction of the me chanical wear and electricalerosion experienced with prior reeds during contact opening and closure,and also prevents contact sticking due to the adhesion that wouldotherwise occur between the gold layers on each reed 1 and 2.

As evidence of the advantages of the invention, we shall compareconventional reed switches in which the contact surfaces are plated onlywith gold, with make-type reed switches constructed in accordance withthe invention and having the contact portion of each reed made of anickel-iron alloy plated with nickel or cooper and then with gold. Theswitches under consideration might have typical operatingcharacteristics as follows: a contact pressure of 5 grams, a restoringforce of 5 grams, a normally open gap of 0.1 mm., and a natural reedfrequency of 2.4 kc./s.

Curves A and A in FIG. 4 show cumulative distribution curves for thecontact resistances of such reed switches made in accordance with theinvention, while .curve B shows the contact resistances for conventionalreed switches of the same size using gold-diffusion contact surfaces.The improved contact resistance shown by the curves A and A and measuredin milli-ohms is apparent from the graph. The contact resistance wasmeasured at a voltage of 0.9 mv. at 1000 c.p.s. in each case.

FIG. 5 shows the life characteristics of the contact resistances forboth the conventional and the improved reed switches when purelymechanical switching operations are performed, i.e. without currentflowing. In FIG. 5, curve A represents the embodiment of this inventionand curve B represents the conventional reed switch using gold diffusioncontacts. The standard of failure is based upon the contact resistanceexceeding the value of 200 milli-ohms. It will be noted that the resultsof the embodiment utilizing copper as the intermediate layer 6 were soexcellent that no failure occurred and the curve A is therefore notshown.

FIG. 6 shows a comparison of life characteristics of the above-mentionedreed switches when an inductive load is switched on and off repeatedlywithout using a spark quenching circuit. In conducting this experiment,failure of contact separation due to contact bridging was regarded as afault. Curve A shows a case in which an inductive load capable ofstoring energy of 20 millijoules is switched on and off by a reed switchconstructed according to the invention. Curve B shows a case in whichthe same circuit is switched on and off by a con-- ventional reed switchwith the well knowngold diffusion contacts. Curve B shows a case inwhich an inductive load capable of storing energy of 2 millijoules isswitched on and off by a conventional reed switch. Where an inductiveload capable of storing energy of 2 millijoules is switched on and offrepeatedly by a reed switch constructed in accordance with theinvention, it was found that no faults occurred until approximately 20million operations.

The multi-layer diffused contact copper-gold construction is superiorfor very low current working conditions, while the multi-layer diffusedcontact nickel-gold construction shows superiority when used under ratedcurrent working conditions.

From the foregoing it will be appreciated that the teaching of theinvention makes possible switches of greatly superior characteristicssuch as lower contact resistance, lower failure rate, less contactsticking and greater reliability.

Although this invention has been described above in connection with themake-type reed switch as a specific embodiment of the invention, it willbe understood that the invention is also applicable to the transfer typesealed metallic contact device which is commonly referred to 'as thetransfer type reed switch, and other switches as well.

While the foregoing description sets forth the principles of theinvention in connection with specific apparatus, it is to be understoodthat the description is made only by way of example and not as alimitation of the scope of the invention as set forth in the objectsthereof and in the accompanying claims.

What is claimed is:

1. A reed type switch having at least one reed actuable in response to amagnetic field, and having a contact formed thereon, comprising anelongated member made of an iron-nickel alloy,

an intermediate layer of nickel on one end of said member,

and a layer of gold on said intermediate layer,

a portion of metal from said intermediate layer being diffused into saidgold layer, and a portion of metal from said gold layer being diffusedinto'said intermediate layer and into said member, whereby said contactis provided on said one end of said member, said contact having theimproved characteristics of improved reliability, longer life and lowercontact resistance.

2. A reed type switch having at least one reed actuable in response to amagnetic field, and having a contact formed thereon, comprising anelongated member made of an iron-nickel alloy,

an intermediate layer of copper on one end of said member,

and a layer of gold on said intermediate layer,

a portion of metal from said intermediate layer being diffused into saidgold layer, and a portion of metal from said gold layer being diffusedinto said intermediate layer and into said member, whereby said contactis provided on said one end of said member, said contact having theimproved characteristics of improved reliability, longer life and lowercontact resistance.

3. A reed type switch having at least one reed actuable in response to amagnetic field, and having a contact formed thereon, comprising anelongated member made of an iron-nickel alloy,

an intermediate layer on one end of said member, said layer being of ametal selected from the group consisting of nickel, copper, cobalt andmanganese,

and a layer of gold on said intermediate layer,

a portion of the metal from said intermediate layer being diffused intosaid gold layer, and a portion of the metal from said gold layer beingdiffused into said intermediate layer and into said member, whereby saidcontact is provided on said one end of said member.

References Cited by the Examiner UNITED STATES PATENTS 2,653,199 9/1953Brown et al 200-166 2,812,406 11/1957 Egan 200-166 3,162,512 12/1964Robinson 29-194 X KATHLEEN H. CLAFFY, Primary Examiner.

BERNARD A. GILHEANY, Examiner.

1. A REED TYPE SWITCH HAVING AT LEAST ONE REED ACTUABLE IN RESPONSE TO AMAGNETIC FIELD, AND HAVING A CONTACT FORMED THEREON, COMPRISING ANELONGATED MEMBER MADE OF AN IRON-NICKEL ALLOY, AN INTERMEDIATE LAYER OFNICKEL ON ONE END OF SAID MEMBER, AND A LAYER OF GOLD ON SAIDINTERMEDIATE LAYER, A PORTION OF METAL FROM SAID INTERMEDIATE LAYER,DIFFUSED INTO SAID GOLD LAYER, AND A PORTION OF METAL